High temperature thermal processes, for example, generation of steam for the production of electricity in power plants utilizing fossil fuels, often create environmentally harmful by-products. These compounds, including nitrogen oxides (NOx) and sulfur dioxide (SO2) must be removed from the flue gases of the high temperature thermal processes before the gases are discharged into the environment, for example before exiting the power plant and contacting the environment.
The standard for removing nitrous oxides from flue gases is the selective catalytic reduction (SCR) process using an SCR catalyst (also called a DeNOx catalyst), where a reducing agent, typically ammonia, is injected and mixed into the flue gases, and sent through a catalytic reaction chamber where the catalyst facilitates the reduction of NOx using the reducing agent to form elemental nitrogen (N2) and water.
Desulphurization of the flue gas, for example removal of SO2, may be carried out by applying known methods in which the SO2 produced in the combustion process is oxidized to SO3. This is done prior to exposure of the flue gases to the reduction catalyst. The SO3 may then be absorbed into alkaline solution and removed from the process, usually in the form of gypsum.
The flue gases from the combustion processes also typically contain fly ash particulates formed during the combustion process. Fly ash and other particulates may accumulate in the SCR catalyst or in or on various components of an SCR system. Removal of fly ash from the flue gas may involve various technologies depending on the physical properties of the fly ash. The physical properties of the fly ash varies depending on the fuel type and the operating conditions in the thermal processes. The fly ash can range from a fine powder to Large Particle Ash (LPA also known as “popcorn ash”, from about 0.1 cm to about 2.5 cm) and can develop into large chunky pieces (from about 2.5 cm to about 13 cm or even larger) when it accumulates in or on the SCR catalyst surface and passageways or on components of an SCR system. The various types of fly ash form in the boiler and easily carry over into the SCR reactor causing accumulation and plugging of the various components of the SCR system, which can lead to one or more of the following: maldistribution of the flue gas, loss of catalytic performance through loss of available DeNOx potential, unacceptable NH3 slip, excessive pressure drop and catalyst erosion damage. Fine powder fly ash may be removed using Electro Static Precipitators (ESP), which are typically installed upstream and/or downstream of the SCR system depending on the SCR arrangement (i.e., high dust, low dust or tail end arrangement). The LPA, also known as popcorn ash, can be collected prior to the SCR reactor by means of LPA screens, which are typically located in the flue gas stream between the economizer outlet and the SCR inlet.
Despite the above mentioned technologies, the fly ash removal may not be sufficient to protect the catalyst or the various components of the SCR system from plugging by or accumulation of fly ash particulates, which can lead to premature loss of SCR performance. For example, loose powder can plug channels of honeycomb-type and corrugated-type catalysts with individual channels becoming partially or fully inaccessible to flue gas. Furthermore, chunky fly ash particulates and LPA can deposit on top of the catalyst module or on other components of the system, blocking the flue gas passage through honeycomb-, plate-, or corrugated-type SCR catalyst modules and access to the catalytic surfaces. Popcorn ash can travel into the channels of honeycomb, corrugated, or plate SCR catalysts and deposit in the channel where it can become wedged between the channel walls, blocking flue gas flow and providing an environment for further fly ash particulates to accumulate and plug the channel. The result can be a catalyst with pluggage ranging from 5% to 100% and reduced NOx removal efficiency.
In addition, it is generally know in the regeneration of SCR catalysts that the physical cleaning of the catalyst to remove any loose fly ash accumulated on the module frame, box frame, on top of the catalyst and within the various passageways of the catalyst, for example plugs within the catalyst channels, is an important step prior to a subsequent wet-chemical based regeneration process. Removal of fly ash plugs prior to wet-chemical regeneration ensures that loose fly ash particulates are not carried into the treatment tanks or accumulate in the chemical solutions used during the regeneration processes where the fly ash particulates could potentially cause problems, such as plugged equipment, damaged equipment due to the abrasive effects of the fly ash and a reduced effectiveness of the chemicals in the process. Therefore, reducing the amount of loose fly ash particulates results in decreased discharge rates of chemical solutions, savings in chemical solutions, and preventing mechanical failures due to abrasive corrosion. Further, removal of the fly ash prior to wet-chemical treatment may also decrease the accumulation of catalyst poisons, such as iron, in the treatment tanks.
SCR catalyst structures, such as honeycomb, plate, and corrugated catalyst are typically dry cleaned using vacuuming, blowing with compressed air, or manually cleaned using scrapers and poking devices of various shapes and forms. The SCR catalysts may also be pressure washed to remove fly ash plugs. However, pressure washing of the catalyst can dissolve catalyst poisons present in the fly ash (e.g., iron) and deposit them on the catalyst surface or surface of other components in the SCR system. Further, water from pressure washing may react with SO3 on the catalyst or in the fly ash to form sulfuric acid (H2SO4), which is corrosive to the module frame and plate catalyst support material surfaces and can result in further liberation of iron as the module is left to dry in the environment. Water from pressure washing may also cause fly ash to harden within channels and in between plates if left to dry.
Thus, there remains a need for additional and effective dry physical cleaning methods to not only remove fly ash from an SCR catalyst and system, but also to open and unplug catalyst channels and provide an accessible catalyst surface prior to a wet-chemical rejuvenation or regeneration process. Further, there is a need for alternative fly ash removal methods that can be applied to the SCR catalyst in situ, when the catalyst is still installed on-site in the SCR reactor, or ex situ, where the catalyst module is removed from the reactor and treated either on-site or at a regeneration facility.